A block copolymer has a molecular structure in which polymer blocks having different chemical structures are linked by covalent bonds. The block copolymer may form a periodically arranged structure such as a sphere, a cylinder or a lamella through phase separation. The size of a domain of the structure formed by self assembly of the block copolymer may be controlled in a wide range and it is possible various shapes of structures and therefore the block copolymer can be used, for example, in a pattern forming by a lithography, or magnetic recording media or various next generation nano device such as high density magnetic recording media, a nanowire line preparation, quantum dot or a metal dot.